Apparently the reason German engineers designed their tanks with flat armor was because of a belief among engineers that sloping the armored plates too much weakened the structure of the design (05:10 min, https://www.youtube.com/watch?v=w3oUeIZ5ieU).
Was this the reasoning among the other countries engineers aswell for the choice of using flat armor?
The KV-1 for example has only slightly sloping front armor, while the T-34, which isn't designed to resist as heavy hits, has much heavier sloping armor.
Were their reasoning correct with the days technology or were they wrong? If so, why didn't they simply do some tests which may have put their assumptions to the test?
The other answers for the reasoning behind flat armor are difficult to understand, at least to me, considering how many advantages there are to sloping armor (at least on the front) and disadvantages to flat armor (weaker protection, more complicated and less sturdy design, heavier weight).
The Germans were no strangers to sloped armour. Look at the Sd.Kfz.221 for instance. It features quite highly sloped armour plates, with the maximum thickness being 14.5 mm, the same as on early German tanks.
German tank design followed a different philosophy, however. The tank hulls were made up of two parts: the tub (Wanne) containing the engine, drivetrain, etc, and the superstructure (Aufbau) with the roof, hull armament (if any), observation ports, etc. This was a very convenient arrangement since in order to create a specialized vehicle like an assault gun all you had to do was replace the superstructure. This also made repairs easier since you could just swap a damaged superstructure for an intact one from a tank where the tub or turret were damaged beyond repair. It is also easier to install vision ports and machine gun balls into vertical armour than sloped, so this arrangement was considered acceptable. Since it was possible to achieve desired protection within the given weight class, there was no need to radically change the layout.
I would not describe the use of unsloped armour as "wrong". As long as it was possible to use techniques that were already mastered in production to meet requirements, there is no reason for radical change, especially if it requires changing the design of lots of other elements that are already in production.
Sources:
http://www.tankarchives.ca/2017/01/pzkpfwiv-ausf-through-c.html